Superconducting Pb Island Nanostructures Studied by Scanning Tunneling Microscopy and Spectroscopy

Superconductivity of nanosized Pb-island structures whose radius is 0.8 to 2.5 times their coherence length was studied under magnetic fields using low-temperature scanning tunneling microscopy and spectroscopy. Spatial profiles of superconductivity were obtained by conductance measurements at zero-bias voltage. Critical magnetic fields for vortex penetration and expulsion and for superconductivity breaking were measured for each island. The critical fields depending on the lateral size of the islands and existence of the minimum lateral size for vortex formation were observed.

Figure 1

(a) STM image of a 9 ML Pb island. (b),(c) Tunneling spectra showing a superconducting gap measured at the center $A$ (b) and a peripheral site $B$ (40 nm from $A$) (c) under various magnetic fields perpendicular to the island. (d) Plot of zero-bias conductance (ZBC) measured under various magnetic fields. (e) ZBC plot measured at $A$ near the magnetic field where the ZBC jumps at the island center.

Figure 2

(a) STM image of a Pb island, same as that in Fig. 1, showing an area where the following ZBC images were taken. (b)–(e) ZBC images taken under magnetic fields of 0.0 T (b), 0.5 T (c), 0.6 T (d), and 1.0 T (e). (f) Cross-sectional ZBC profiles averaged in the rectangular area shown in Fig. 2d. The calculated profile of local density of states at the Fermi level for a vortex is also drawn with a solid line.

Figure 3

ZBC plots and images obtained on Pb islands with various sizes. (a) Pb island (radius: $\sim 30\mathrm{nm}$) smaller than that in Fig. 1. (b) Pb island (radius: $\sim 75\mathrm{nm}$) larger than that in Fig. 1. (c) Elongated Pb island with thicknesses of 11 and 12 ML.

Figure 4

Lateral size dependence of various critical fields: transition field between superconducting and normal states $H_{\mathrm{SN}}$, vortex penetration $H_{p1}$, and vortex expulsion $H_{\mathrm{ex}1}$. All critical fields were obtained on 9-ML-thickness Pb islands. The broad vertical line indicates the critical size for vortex formation.